Position sensor, designed in particular for detecting a steering column torsion

ABSTRACT

A position sensor, designed in particular for detecting a steering column torsion, including a first magnetic structure including a plurality of magnets and a second magnetic structure including two ferromagnetic rings having a plurality of teeth and defining an air gap. At least a magneto-sensitive element is placed in the air gap. The first and second magnetic structures are respectively integral with two parts in relative rotation. The two ferromagnetic rings are nested and have each a substantially tubular part forming axially oriented teeth connected by a flux-closing zone, the detecting air gap being delimited by the flux-closing zones.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the art of position sensors, and moreparticularly to position sensors intended to measure the torsion of asteering column, although such an application is not exclusive.

2. Description of the Related Art

In the prior art there is known U.S. Pat. No. 4,984,474, which describesa prior art sensor provided with a stator part comprising aferromagnetic piece forming radial teeth at two levels, disposed facingmulti-pole magnets that are radially magnetized in alternatingdirections.

An additional ferromagnetic piece is disposed facing the stator part,and forms an air gap in which there is placed a Hall probe.

This prior art solution is not satisfactory, because it leads to a lossof magnetic signal between the stator part and the part containing theHall probe. Furthermore, the magnetic field generated by the magnetsleads to losses due to the sensor structure.

Also known in the prior art is a sensor described in U.S. Pat. No.4,784,002, which describes another position sensor comprising a partprovided with a plurality of axially oriented magnets cooperating withradial teeth of a stator part.

This structure also leads to magnetic leaks and to reduced efficiency,manifested by a poor “signal-to-noise” ratio.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to overcome these disadvantagesby providing an improved position sensor with better signal-to-noiseratio.

Another object of the invention is to reduce the radial spacerequirement.

To this end, the invention relates in its most general concept to aposition sensor, intended in particular for detection of the torsion ofa steering column, comprising a first magnetic structure containing aplurality of radially magnetized magnets and a second magnetic structurecontaining two ferromagnetic rings provided with a plurality of teethand defining an air gap, in which there is placed at least onemagnetosensitive element, the two magnetic structures being integralrespectively with two parts in relative rotation, characterized in thatthe two ferromagnetic rings are intermeshed and each is provided with asubstantially tubular part forming axially oriented teeth, connected bya transverse flux-closure zone, the detecting air gap being bounded bythe said flux-closure zones.

The first magnetic structure is advantageously composed of aferromagnetic tubular yoke provided with a plurality of tangentialnotches, in which there are seated thin magnets magnetized substantiallyradially in identical directions.

According to a preferred embodiment, the height of the teeth correspondssubstantially to the height of the magnets. According to an alternativeembodiment, the first and second magnetic structures are movablerelative to the magnetosensitive element.

According to a special embodiment, the position sensor is provided withN magnetosensitive elements, N corresponding to the number of phases ofa brushless DC motor whose movement is controlled by the said sensor.

According to a first embodiment, the rings are provided withflux-closure zones having the shape of disks.

According to a second embodiment, the rings are provided withflux-closure zones having the shape of half-toruses.

According to a third embodiment, the rings are provided withflux-closure zones cut to form a plurality of teeth.

According to another embodiment, the rings are provided withflux-closure zones extending over 360° C.

According to another alternative embodiment, the rings are provided withflux-closure zones extending over an annular sector correspondingsubstantially to the dimension of the magnetosensitive element.

The invention also relates to a torsion sensor comprising two rotatingparts connected by an elastic test member, and a position sensorcomprising two parts integral respectively with the said rotating parts,the position sensor being composed of a first magnetic structurecontaining a plurality of radially magnetized magnets and a secondmagnetic structure containing two ferromagnetic rings provided with aplurality of teeth and defining an air gap, in which there is placed atleast one magnetosensitive element, the two magnetic structures beingintegral respectively with two parts in relative rotation, characterizedin that the two ferromagnetic rings are intermeshed and each is providedwith a substantially tubular part forming axially oriented teeth,connected by a transverse flux-closure zone, the detecting air gap beingbounded by the said flux-closure zones.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention will be better understood by reading thedescription hereinafter with reference to the attached drawingspertaining to a non-limitative embodiment, wherein:

FIG. 1 illustrates a schematic view of a steering column;

FIG. 2 illustrates an exploded view of a first practical example of asensor;

FIG. 3 illustrates a view of the second structure of the said sensor;

FIG. 4 illustrates an enlarged view, in partial section, of the sensor;

FIG. 5 illustrates an exploded view of a second embodiment;

FIG. 6 illustrates the response curve of the sensor according to FIG. 5;

FIG. 7 illustrates another alternative embodiment (fixed probe and fixedstator);

FIG. 8 illustrates a cross-sectional view;

FIG. 9 illustrates an alternative embodiment of the invention in whichthe detecting air gap is disposed between two fixed elements.

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention is to overcome these problems of lowsensitivities and it relates to contactless position sensors intendedfor the measurement of angles similar to or smaller than 10° C., inapplications such as steering-column torque sensors, for example (thesignal then will be processed to provide steering assistance). Theangular position sensor described hereinafter is intended for themeasurement of a very small angular difference (a few degrees) betweentwo shafts connected by a torsion bar. Such an application for torquemeasurement is described in FIG. 1. In the range of linear deformationof this torsion bar, this angular difference (α1-α2) will beproportional to the torque applied between the two shafts (1, 3)connected by an elastically deformable test member (2). The measurementof this angular difference by the sensor will allow an electrical signalproportional to the applied torque to be delivered at the output of themagnetosensitive element. In the case of the steering-column torquesensor, the sensor (4) must also permit measurement of the angulardifference between two shafts turning relative to the fixed frame ofreference represented by the passenger compartment of the vehicle. Thismeans that α₁ and α₂ are angles that can be larger than 360° (thesteering column can execute several turns). The angular measurement musttherefore take place between the two shafts (1, 3) when the torsion bar(2) is deformed, each of the two shafts being freely rotatable throughseveral turns. A typical torsional working angle in this application isfrom ±2° to at most ±4°. It is therefore evident that the problemconsists of providing on the one hand a highly sensitive position sensorand on the other hand a system with which the magnetosensitive elementcan be fixed relative to the passenger compartment as the frame ofreference.

FIG. 2 illustrates an exploded view of a first practical example of asensor according to the invention.

It is composed of a first magnetic structure (5) and of second magneticstructure formed by two intermeshing rings (6, 7). The two magneticstructures have tubular general shape and are coaxial.

The first magnetic structure (5) is formed by a yoke (8) of tubularshape provided with cavities for seating a plurality of thin magnets (9)magnetized in radial direction, or in a direction parallel to the radialdirection and passing through the center of the magnet.

These magnets are embedded in a cavity having a thickness of between 0.2and 0.9 times that of the magnet.

The magnets are separated by angular sectors (10) of the yoke.

The second structure is formed by two ferromagnetic rings (6, 7)provided with teeth (11, 12) that extend axially and that are separatedby open intervals allowing intermeshing with the teeth of the oppositering.

The teeth are prolonged by respective flux-closure zones (13, 14)extending generally in a transverse plane, perpendicular to the mainorientation of the teeth.

These two flux-closure zones bound an annular air gap (16) in whichthere is positioned a magnetosensitive element (15).

FIG. 3 illustrates a view of the second structure in assembledcondition, without the first structure, which is now lodged in thecentral cavity, and FIG. 4 shows a view in detail and in section of thesaid sensor.

The first structure is provided with N magnets (9), and each of therings of the second structure has N teeth. The magnetosensitive element(15), a programmable Hall-effect probe, for example, is fixed relativeto the fixed frame of reference corresponding to the passengercompartment. It is placed in the air gap (16) between the twoferromagnetic collectors (13, 14), each of which has collected the fluxof N teeth, and in such a way as to allow the two collars to turnthrough several turns.

Each of the structures can rotate relative to the frame of reference ofthe passenger compartment, and exhibits a differential movement of a fewdegrees relative to the other as a function of the applied torque, whichwill be manifested by a flux variation of a few hundred Gauss in therotating air-gap (16). The analog signal emitted by the Hall probe (15)will therefore deliver an electrical image of the torque applied betweenthe two shafts supporting the stator (6, 7) on the one hand and therotor (5) on the other.

In the case of steering-column torque sensors, the torque information isgenerally processed so as to drive an electric motor of the brushless DCtype (BLDC). The action of this electric motor will be to provideelectrical steering assistance, by delivering a torque proportional tothat detected by the torque sensor, while following a positionproportional to that of the steering column. Such motors generally havethree windings known as “phases”, offset by an electrical angle of 120°.Rotation of these three-phase motors is assured by a controller, whichwill generate three sinusoidal signals of amplitude proportional to thetorque delivered by the torque sensor, while following a positionproportional to that of the steering column. In general, these twotorque and position signals are obtained from two different sensors.

According to the invention described in FIG. 5, the magnetic collectors(13, 14) can be toothed and can have D teeth (19, 20) over 360°. Amagnetosensitive element (15) placed in the air gap (16) of FIG. 5 willtherefore sense an alternating magnetic field, whose period isproportional to D and to the position of the “stator” part (5) which isrotating relative to the fixed frame of reference of the passengercompartment (but is a stator relative to the rotor (6, 7)), and is alsoproportional to the torque exerted between (5) and (6, 7).

If three magnetosensitive elements (21, 22, 23) spaced apart by a poleoffset equivalent to an electrical period of 120° are placed in the airgap (16), there is obtained at the output of these threemagnetosensitive elements the three sinusoidal curves described in FIG.6, the amplitude of which is proportional to the torque exerted on thesteering column, and which at the same time yield information on theposition of the steering column.

If the number D of teeth is chosen judiciously as a function of thereduction ratio R that is often associated with the BLDC motor, thesetwo combined signals can be used directly to drive the BLDC motor via atransistorized power module.

FIG. 7 illustrates another alternative embodiment, in which the ringsare provided with two flux-closure zones reduced to reduced angularsectors (30, 31), whose dimensions correspond substantially to thedimensions of the Hall probe (15).

The principle described hereinabove is not limited to applications as asteering-column torque sensor but can also be applied to measurements ofvery small angles, such as applications as a brake-pedal oraccelerator-pedal sensor. In fact, it is possible to imagine the twoferromagnetic collectors (13, 14) as not extending over 360° but asbeing limited to a few dozen degrees, as indicated in FIG. 7.

FIG. 8 illustrates a cross-sectional view of the sensor.

The alternative structure illustrated in FIG. 9 was developed with theobjective of creating the detecting air gap (16) between two fixedelements (34, 35).

In the same way as in the structures illustrated in the precedingfigures, a variation of induction is created in the teeth (11, 12) by anangular phase shift between the first magnetic structure, or in otherwords the rotor (5), and two intermeshed magnetic structures, which inthis case are toothed pieces (32, 33). The magnetic circuit is thenprolonged by fixed elements (34, 35) separated from the magneticstructures (32, 33) by a mechanical gap (41). Thus, in this alternative,the rings (6, 7) are therefore composed of two movable toothed pieces(32, 33) and two fixed elements (34, 35).

The two fixed elements (34, 35) are composed of two flux-integrationzones (36, 37) that completely (angle of 360°) or partly surround thetoothed pieces (32, 33), and of two magnetic-flux concentrators (38,39), which create a detecting air gap (16) in which there are insertedthe magnetosensitive element or elements (15, 40).

1. A position sensor, comprising: a first magnetic structure containinga plurality of magnets; a second magnetic structure containing twoferromagnetic rings provided with a plurality of teeth and defining anair gap; at least one magnetosensitive element placed in the air gap;the first and second magnetic structures being integral respectivelywith two parts in relative rotation, wherein the two ferromagnetic ringsare intermeshed and each is provided with a substantially tubular partforming axially oriented teeth, connected by a flux-closure zone, theair gap being bounded by the flux-closure zones.
 2. A position sensoraccording to claim 1, wherein the position sensor is configured fordetecting torsion of a steering column.
 3. A position sensor accordingto claim 1, wherein the first magnetic structure is composed of aferromagnetic tubular yoke provided with a plurality of tangentialnotches in which are seated thin magnets magnetized substantiallyradially in identical directions.
 4. A position sensor according toclaim 3, wherein the thin magnets are in a form of radially magnetizedtiles.
 5. A position sensor according to claim 3, wherein the thinmagnets are in a form of parallelepiped magnets magnetized in adirection perpendicular to a plane of a main face.
 6. A position sensoraccording to claim 1, wherein a height of the teeth correspondssubstantially to a height of the plurality of magnets.
 7. A positionsensor according to claim 1, wherein at least one of the first andsecond magnetic structures is movable relative to the at least onemagnetosensitive element.
 8. A position sensor according to claim 1,wherein the at least one magnetorestrictive element comprises Nmagnetosensitive elements, N corresponding to a number of phases of abrushless DC motor whose movement is controlled by the position sensor.9. A position sensor according to claim 1, wherein the two ferromagneticrings are provided with flux-closure zones having a shape of transversedisks.
 10. A position sensor according to claim 1, wherein the twoferromagnetic rings are provided with flux-closure zones having a shapeof half-toruses.
 11. A position sensor according to claim 1, wherein thetwo ferromagnetic rings are provided with flux-closure zones of tubularshape.
 12. A position sensor according to claim 1, wherein the twoferromagnetic rings are provided with flux-closure zones cut to form aplurality of teeth.
 13. A position sensor according to claim 1, whereinthe two ferromagnetic rings are provided with flux-closure zonesextending over 360° C.
 14. A position sensor according to claim 1,wherein the two ferromagnetic rings are provided with flux-closure zonesextending over an annular sector corresponding substantially to adimension of the magnetosensitive element.
 15. A position sensoraccording to claim 1, wherein the two ferromagnetic rings are composedof two movable toothed pieces and two fixed elements.
 16. A torsionsensor comprising: two rotating parts connected by an elastic testmember; a position sensor comprising two parts integral respectivelywith the two rotating parts, the position sensor being composed of afirst magnetic structure containing a plurality of radially magnetizedmagnets and a second magnetic structure containing two ferromagneticrings provided with a plurality of teeth and defining an air gap, atleast one magnetosensitive element placed in the air gap, the first andsecond magnetic structures being integral respectively with two parts inrelative rotation, wherein the two ferromagnetic rings are intermeshedand each is provided with a substantially tubular part forming axiallyoriented teeth, connected by a transverse flux-closure zone, the air gapbeing bounded by the flux-closure zones.